Normality of links on a spare path or the links and nodes is monitored. When the spare path cannot be used, at least any one of nodes at both ends in a segmented path section is changed to another node on a working path. The segmented path section is changed to set another spare path. When a fault monitoring section which can accumulatively detect abnormal information is changed to the changed partial working path section, alarm generation in the fault monitoring section is prevented to release the prevention after completion of the change.